Fix delegate types in manual README examples

[dajiaohuang]

Problem

Both manually composed examples in How to use create a non-generic LambdaExpression, then call CompileFast() and invoke the inferred result as f(new B()). The non-generic overload returns Delegate, so copying either example fails with compiler error CS0149 (Method name expected).

Change

Specify CompileFast<Func<B, X>>() in both examples, matching the existing ManuallyComposedExprTests.Should_compile_manually_composed_expr_with_parameters usage. No API or behavior changes.

Validation

• Reproduced CS0149 with the original example and simple A/B/X types plus the constructor declaration.
• Both corrected README snippets compiled and ran against NuGet 6.0.0 on .NET 10, using both System.Linq.Expressions and the documented LightExpression alternative (four cases). Checked that the constructed X retains the original A and supplied B.
• Corrected example also passed against the current source project.
• No full library test matrix run for this documentation-only change.

[dadhi]

Thanks for finding - but the fixes should go into the Expression.Lambda